1. Field of the Invention
The present invention relates to the control of bidirectional switches of medium power, such as, for example, triacs.
2. Discussion of the Related Art
A bidirectional switch includes two main terminals A1 and A2 and a gate G. A bidirectional switch capable of switching on when a positive or negative voltage exists between terminals A1 and A2 and a current pulsexe2x80x94negative or positivexe2x80x94is made to flow between the control terminal and terminal A1 that will be called the reference terminal. The bidirectional switch then remains conductive until the current flowing therethrough crosses zero.
In many cases, it is desired to only allow a bidirectional switch to turn on when the voltage across its main terminals is close to zero. This type of control is called a zero crossing control circuit although, in fact, it occurs when the voltage across the bidirectional switch is sufficient to enable its turning on, for example about ten volts.
Several known circuits implement this function. FIG. 1 illustrates such a zero crossing control circuit described in European patent application No. 0837545 which is incorporated herein by reference. A bidirectional switch TR is connected by a main terminal A2 to a load L, the series connection of load L with bidirectional switch TR being connected across A.C. voltage terminals I1, I2, for example, the mains. Terminal I1 is at a reference potential, for example the ground, and is connected to main terminal A1, that is, the reference terminal of the bidirectional switch.
Two complementary transistors are connected between gate G and reference terminal A1 of bidirectional switch TR. These complementary transistors are an NPN-type bipolar transistor Q1 and a PNP-type bipolar transistor Q2. The emitter of transistor Q1 and the collector of transistor Q2 are connected to gate G. The collector of transistor Q1 and the emitter of transistor Q2 are connected to terminal A1 of bidirectional switch TR. The bases of transistors Q1 and Q2 are connected to each other and to terminal A2 via a resistor of high value R1. The control order is applied between terminals I3 and I4. Terminal 14 is connected to terminal I1 and forms a reference terminal. Terminal I3 is connected to gate G via a resistor R2. The control order is a signal having a 0-volt value (the potential of terminals I1 and I4) when the bidirectional switch is not desired to turn on and a negative value, for example xe2x88x925 volts, when the bidirectional is desired to be turned on.
The circuit operates as follows.
When the voltage on terminal I2 is high, one of transistors Q1 or Q2 is controlled to be turned on. Terminals G and A1 are then short-circuited by one of the transistors and no control current can flow between terminals G and A1. Bidirectional switch TR is thus off.
When the voltage on terminal I2 is smaller than a given threshold, both transistors Q1 and Q2 are off and, if the voltage on control terminal I3 is negative, a current will flow from terminal A1 to terminal G and will turn bidirectional switch TR on. Thus, the application of a control order (negative voltage) on terminal I3 can be considered to be delayed until the voltage on terminal A2 has fallen to a low value with respect to the voltage on terminal A1.
Then, bidirectional switch TR will turn off each time the voltage thereacross falls and becomes close to a zero value. The bidirectional switch will then be turned on again at the beginning of the next (positive or negative) halfwave if the control order is still present.
A disadvantage of this type of circuit is the fact that its positive and negative switching thresholds, that is, the voltage difference across transistors Q1 and Q2 beyond which one of them is off, are not precisely known. Indeed, these thresholds, on the order of a few volts, typically approximately 10 volts, depend in particular on the value of resistor R1 and on the gain of the transistors. Now, on the one hand, it is relatively complex to obtain complementary transistors Q1 and Q2 with identical gains. On the other hand, the gains of each of the transistors will vary during their lifetime, especially according to temperature. A dispersion of the value of the maximum positive and negative voltages beyond which the bidirectional switch is inhibited can thus be experimentally observed.
An object of the present invention is to provide a novel monolithic structure of a zero crossing control circuit for a bidirectional switch that enables stabilizing the value of the voltage beyond which the bidirectional switch is inhibited.
Another object of the present invention is to provide such a monolithic structure in which the positive and negative inhibition thresholds are identical.
Another object of the present invention is to provide monolithic embodiments of such control circuits.
To achieve these and other objects, the present invention provides a monolithic implementation of a zero crossing control circuit of a bidirectional switch including two transistors of complementary types connected in parallel between the gate of the bidirectional switch and the main reference terminal of the bidirectional switch, the gate of the bidirectional switch being connected to a control source via a first resistor, and each of the control terminals of the transistors being connected to the second main terminal of the bidirectional switch via a second resistor of high value, a zener diode being interposed between the second resistor and each of the control terminals according to a biasing adapted to turning on each of the transistors when the zener threshold is exceeded. According to the present invention, the circuit is formed in the same semiconductor substrate of a first conductivity type as the bidirectional switch.
According to an embodiment of the present invention, the control terminals of the transistors are interconnected and the zener diodes are series connected, anode to anode, between the second resistor and the interconnection node of the control terminals.
According to an embodiment of the present invention, the second resistor is formed in the same first portion of the substrate as the bidirectional switch.
According to an embodiment of the present invention, the zener diodes are formed on the front surface side of the substrate in a layer of the second conductivity type, including, on the front surface side, two regions of the first conductivity type contacted by metallizations, and a region being formed, by deep diffusion from the front surface, under and in contact with the layer.
According to an embodiment of the present invention, a first zener diode is formed in the same portion of the substrate as one of the complementary transistors.
According to an embodiment of the present invention, a second zener diode is formed in a portion of the substrate distinct from a portion where the second transistor is formed.
According to an embodiment of the present invention, a second zener diode is formed in a same portion of the substrate as the second transistor.
According to an embodiment of the present invention, the two complementary transistors are of bipolar type.
According to an embodiment of the present invention, the two complementary transistors are of MOS type.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.